I'd just get an ICS either with or without RIS. I personally wouldn't be too fussed about getting any external parts other than RIS add-ons and optics, so the fact that ICS have bad compatibility with body kits, grips, barrels and that sort of stuff wouldn't bother me.
The guns are solid, they shoot nice, they're high quality, they're the easiest M4s to work on by a country mile and they have the heft and strong build of a real one to boot.
Additionally, the internals are spot on in general, I wouldn't even bother doing anything substantial to the gearbox, just stick a tightbore in it, new hop, maybe a new nub. Shim the gears, regrease the box, potentially swap out the motor and you've got a A* fantastic M4.